Indian Journal of Pure & Applied Physics Vol. 48, October 2010, pp. 723-730 Characterization of low pressure plasma-dc glow discharges (Ar, SF 6 and SF 6 /He) for Si etching Bahaa T Chiad a, Thair L Al-zubaydi b, Mohammad K Khalaf a & Ausama I Khudiar c a Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq b Department of Materials Science, Ministry of Science and Technology, Baghdad, Iraq c Department of Laser and Optoelectronics, Ministry of Science and Technology, Baghdad, Iraq E-mail: ausamaikhudiar@yahoo.com Received 26 June 2009; revised 14 July 2010; accepted 12 August 2010 Low-pressure plasma reactor which is generated for SF6, SF6/He and Ar gases discharges between two metal electrodes (planer parallel) using dc-high voltage power supply of 2 kv has been proposed. Paschen s curves show the breakdown voltage of gases as a function of the parameter p*d which is the product of the pressure in the chamber (P=6.5 10 2-1.5 10 1 mbar) and the distance between the two electrodes (d=4.6 cm). The minimum breakdown voltages were found 450 V at pressure of 1.35 10 1 mbar and 276 V at pressure of 4.3 10 1 mbar for SF 6 and Ar, respectively. Current-voltage characteristics have been studied at different values of pressure (6.5 10 2-1.5 10 1 mbar) and interelectrodes spacing (3.4, 4.2, 4.6, 5 cm). The SF 6, SF 6 /He and Ar gases discharges plasmas in Si etching have been discussed. Keywords: Plasma etching, Si etching, Dry etching, SF 6 discharge, Glow discharge 1 Introduction Space and laboratory plasmas 1,2 are classified by their electron temperature Te, and charge particle density n. A glow discharge is a kind of plasma consisting of equal concentration of positive and negative charges and a large number of neutral species. The dc-glow discharge plasmas have been used for plasma applications in the low and intermediate pressure regions in modern technology. Cold plasma technologies have been widely used in industry for thin films deposition and surface processing such as sputter- and plasma-assisted chemical vapour deposition. Glow discharge plasma is a tool for heating, sputtering, etching, nitriding and ionization as well as an activator for gaseous atoms and molecules. It is well known that a large number of ions, electrons and excited radicals coexist in plasma. These particles in plasma also have their kinetic energy up to several hundred evs, which may give rise to physical and chemical effects in thin-film formation and surface modification. A wide variety of particles exist in the discharge in addition to ions and electrons, including for example, radicals, excited species, and various fractured gas molecules created by collisions between electronics and gas molecules or atoms. Overall, the discharge system must remain electrically neutral even though some portions of it are not 3,5. One types of processing plasmas that is relevant to this project is low-pressure dc-glow discharges. The simplest configuration employed for striking a gas discharge is a parallel electrode geometry schematically shown in Fig. 1. Two plane metal plates are separated by a distance, d, in a chamber reactor filled with a particular gas at a pressure, p. Breakdown of the gas is achieved by applying an electric field of direct-current (dc) with cathode and anode biased negatively and positively, respectively. When the voltage between the plates is low, the gas is a near-perfect insulator. As the voltage is increased, a small fraction of electrons present in a gas are accelerated towards the anode making collisions with the background atoms. Some of these collisions create positive ions which are then accelerated towards the cathode. When the ions strike the cathode, electrons are liberated from the metal surface as a result of neutralization (secondary electron emission). This process gives rise to an entire avalanche of electrons leading to gas breakdown and discharge formation. The voltage at which breakdown occurs is described by Paschen s law 1,6. The breakdown voltage is found to depend only on the product pd for a given gas and cathode material. Plots of the breakdown voltage versus pd are known as Paschen curves. A characteristic minimum in the function appears at some intermediate value of pd. At low pd values, the breakdown voltage is high because
724 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010 Fig. 1 Plane parallel electrode reactor for producing a dc glow discharge of too few collisions (low pressure or small gap). At high pd values, the breakdown voltage is high because of too many collisions (high pressure or large gap). It should be noted that Paschen curves for different gases or cathode materials will have roughly the same shape but will be shifted from one another. Over the past decades, plasma etching has been widely used in the fabrication of silicon based integrated circuits. However, due to complex physical and chemical effects during etching, issues of reproducibility and control of the interaction processes ultimately limit its widespread application and further progress. In the micro-electronics industry, glow discharge plasmas are often used for etching of surfaces, in order to generate topographical patterns on chips as an alternative to wet chemical etching. Low-temperature plasma processing offers important advantages over wet-chemical methods. First, plasma-processing is dry and safe. Secondly plasma-etching provides finer resolution, sharper etching and less under cutting than can be obtained with chemical ethantes. Thirdly, plasma-processing makes possible to perform sequential etching and stripping operations in the same machine. Finally, plasma-processing creates no pollution problems 1,7. The anisotropy of wet etching of mono characteristic crystalline silicon depends on the crystal orientation. This means that the structuring of the substrate is strongly related to this material feature. When a specific profile is desired wet etching is not appropriate and another process is needed. Plasma etching as shown in Fig. 2, also called dry etching, is a method for structuring the substrate in the gas phase, physically by ion bombardment, chemically by chemical reaction, or by combination of both. Depending on the etching mechanism, isotropic, directional, or vertical etch profiles can be obtained 8,9. Utilizing dry etching, the desired profiles can be generated in polycrystalline as well as in single crystalline and amorphous materials 10. Different etchant gases such as F 2, CF 4, SF 6, NF 3 and CLF 3 as fluoride atoms sources are used in plasma etching. The SF 6 has dielectric strength of about two three Fig. 2 Example of anisotropic (dry-etching) and isotropic (wet etching) Fig. 3 Chamber of dc-glow discharges plasma times that of air. It is non-toxic, non-flammable and doesn t react with other materials because it is inert gas. SF 6 and CF 4 etchants are used in almost different etching processes 11-13. The properties of Si depend strongly on the number of factor such as discharge current, gas used and pressure. It is inevitable that each etching technique with its associated controlling parameters should yield etch samples with different characteristics. 2 Experimental Details A partial ionized plasma source of dc-glow discharge at low pressures has been constructed as home built plasma system, characterized and operated at abnormal glow discharge. The plasma chamber itself is a cylindrical stainless steel vacuum chamber with length and diameter of 50 cm (Fig. 3). Within it, there are two circular electrodes. Both the movable electrode (denoted Anode ) and the fixed electrode
CHIAD et al.: CHARACTERIZATION OF LOW PRESSURE PLASMA 725 (denoted Cathode ) have a diameter of 14.5 cm. Both are made of stainless steel and have exterior high-voltage connections. A front view image of the electrodes is shown in Fig. 4. A system body grounded shield, also made of copper, connected to anode electrode. The feedstock gas is supplied through a gas regulator and kept at a preset pressure by a membrane valve. The experiments are performed using two-stage vacuum pump (mechanical and turbo) to evacuate the plasma chamber down to 10-4 torr. Argon, SF 6 and SF 6 /He are used as plasma gases during measurement to avoid possible negative effect of complex chemical reaction when non-inert gas is applied. Figure 5 shows a photograph of the main experimental set-up used in this work. The I-V, I-P measurements and Paschen's graph refer to dc characterizations of device, the purposes of Fig. 4 Anode and cathode electrode-dark space holder assembly performance analysis and parameter extraction. A typical electrical circuit set-up for forming planar discharges is schematically shown in Fig. 6. The dc characterization set-up essentially consists of power supply (4 kv), digital multimeter (discharge current meter), digital multimeter (cathode voltage meter), pirani-guge and reader. To obtain I-V curves, the discharge was first ignited by providing over potential with the dc power supply. The power supply voltage was then adjusted to vary the discharge current. The main characteristics of plasma discharge such as the breakdown voltage pd, I-V and I-P characteristics depend on the cathode voltage used, the gas pressure and inter-electrode spacing in the chamber. Plasma etch equipment can be divided into two classifications. The first is the parallel-plate diode configured etches system, where the wafers are positioned on the electrode surface. The second classification is the high-pressure plasma etcher where the wafers are positioned in a rack that enables the wafers to float in the plasma. The present work represented the diode-configured system concept 2-3. The etching experiments were performed in a conventional reactive ion etching RIE system with a modified cathode.the cathode set-up used for the dc discharge experiments is shown in Figs 4 and 7. The cathode electrode (14.5 cm diam) was surrounded by a dark space shield also serving as a gas. In order to avoid arcing; a ceramic plate (3mm thick) was inserted between the cathode and the grounded dark space shield. The discharge was truncated by desired masks. This arrangement defines the etching area and masks it possible to calculate the current density. The Si specimens were masked with 1 mm thick desired pattern of lines and spaces and pinhole as shown in Fig. 7. The Si wafer for plasma etching has been cut into pieces (10 10 mm) with 0.5 mm thickness using diamond tip cutter. Silicon substrates are cleaned thoroughly before putting into vacuum chamber. The Fig. 5 Low pressure (dc-glow discharge) plasma system Fig. 6 Electrical circuit used to generate and analyze the glow discharge
726 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010 system is fitted with Ar, SF 6 and SF 6 /He gas sources and is capable of delivering up to 1.3 kv dc voltage. The specimen (wafer) to be etched is placed on cathode surface in the center of the chamber. It is then evacuated to pressures better than 1 10 4 mbar using a mechanical rotary pump and a turbo pump; the etchant gas is then flowed in at a constant rate regulated by flow controllers. The dc power supply is then switched on to start the etching for constant process time (1 h). Argon as discharging gas was injected for cleaning the samples surface for 30 min process time with cathode voltage of 1.3-1.5 kv. The etch rate was calculated as the ratio of the etched depth to the total etching time. Moreover the etched profile was examined with optical microscope. The etch depth of the treated Si samples was determined by using the gravimetric method. Four digital balance was used to found the weight difference between the Si sample which plasma etched and untreated. We can calculate the resulting etch depth by using the following formula: D= m/aρ Fig. 7 Top and side view of the cathode set up where the m is the sputtered material weight, A is the active etched area and ρ is the material (Si) density. 3 Results and Discussion 3.1 Paschen s law graphs The results of our Paschen s law experiments are shown in Fig. 8 for planar parallel electrodes, at fixed Fig. 8 Plot of the breakdown voltage of Ar and SF6 versus pressure and electrode spacing inter-electrode spacing (4.6 cm). The graph shows the minimum voltage that can be expected to produce a sustained glow discharge as a function of the gas pressure (6.5 10 2-5 10 1 mbar) multiplied by the distance between electrodes (4.6 cm). Figure 8 shows the minimum breakdown voltages as 455 V at pressure of 1.35 10 1 mbar and 276 V at pressure of 4.3 10 1 mbar for SF 6 and Ar, respectively. The breakdown voltage can be increased by reducting the glow parameter pd, where further reduction of pressure is not practicable, the same result can be obtained by reducing the effective inter-electrode spacing. The breakdown applied voltage up to Paschen minimum value was found to increase with increasing the pd parameter. These results are related to the dependence of probability on the number of gas molecules between electrodes at fixed temperature, which is formally developed by Paschen s law 1. 3.2 I-V and I-P characteristics The characteristics of glow discharge current/voltage and pressure have been established with the scheme shown in Fig. 9. The current in the external circuit can be measured as a function of the voltage drop between the anode and the cathode. A further decrease in the external current limiting resistance brings the voltage/discharge current characteristic into the abnormal glow discharge region. Since the visible glow already covers the entire work surface, an increase in current density will now be accompanied by an increase in the voltage drop through the resistance of the glow discharge.
CHIAD et al.: CHARACTERIZATION OF LOW PRESSURE PLASMA 727 Fig. 9 I-P characteristic of SF 6 discharges plasmas These positive characteristics and behaviour attributed to the mobility limited version of the Child-Langmuir equation, where the current density is proportional to V 2 and inter-electrode spacing 2-11 (d). The currentpressure (I-P) characteristics can give valuable information about its mechanism. Figure 10 shows discharge currents plotted against pressure using different cathode applied voltages. Breakdown voltage is found to decrease from approximately 1000 volt at (3.7 10 2 mbar) to 650 volt at (5 10 2 mbar). The increment of gas pressure leads to increase of the electron collisions (reduction of free path of electron) with gas molecules, which means increasing the ionization rate. The effective plasma resistance, Reff can be estimated at the various pressures by finding the slope of the I-V plots. From this calculation, Reff is found to decrease from (30) KΩ at 6.5 10 2 mbar to Fig. 10 I-V characteristics of SF 6 discharge plasmas at different value of gas pressure and inter-electrode spacing
728 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010 (8) KΩ at 1.5 10 1 mbar. Evidently, as the pressure is raised the discharge glow column becomes more conductive. The ion and electrons currents are usually interpreted in terms of gas pressure; it is more accurate to consider the gas density. 3.3 Etch results The results discussed below are limited to Si which is widely used in the semiconductors industry today. Gases of SF 6, SF 6 /He and Ar have been tried in reactive plasma process. Typical dc glow discharge I-V curves for the SF 6, SF6/He and Ar etching gases are shown in Fig. 11. The plasma etching photographs of the n-type Si surface are shown in Figs 12-15. The gases pressure and process times were held constant for these experiments. From resulting data of I-V characteristic of plasma discharges for SF 6, SF6 /He, and Ar gases, we found that the current density value of SF6 discharges is maximum in comparison to others for limited area of Si wafer. This result implies that at given pressure and bias potential, the etching rate increases linearly with increase of ion current. From Fig. 12 of dc Ar glow discharge (n-type Si etching photographs) and the weight difference measurements of samples as untreated and plasma treated, we concluded that there is no distinguished etching. This result of dc Ar glow discharge can be used for surface cleaning as primary step to etch processing of other gases. SF 6 plasma were observed to etch Si with high etching depth which attributed to long inherent lifetime for F atoms and their high reactivity with Si (Ref. 12) as expected (Fig. 13). The Fig. 11 I-V characteristics of Ar, SF 6 and different percentage (SF 6 /He) discharges plasmas Fig. 12 Plasma etching of Ar-discharges
CHIAD et al.: CHARACTERIZATION OF LOW PRESSURE PLASMA 729 Fig. 13 Plasma etching of SF 6 discharges results indicated that the etch depth is linearly proportional to the glow discharge current density and the SF 6 gas pressure. The process of SF 6 discharges gives an etching profile with good agreement images of masks. The resultants provide sharper etching and more resolution than obtained with chemical etchants 14. The He gas was mixed with SF6 in order to sustain a uniform and stable plasma. The n-type Si etching rate increases for low He concentration, as shown in Fig. 14(a,b) for SF 6 (90)/He(10) and SF 6 (80)/He(20), while etch rate becomes very low at high He concentration of 40%. The observations of Fig. 14 indicated that the etched depth has been improved for the calculated etch rate of 0.1-0.2 µm/min. The etch rate depends on the free fluoride at low concentrations of He, as SF6 pressure increases the etch depth will also increase. The same results are obtained for Si etch rate as mixing of CF 3 Br and He Fig. 14 Plasma etching of SF 6 /He discharges (low He percentage) Fig. 15 Plasma etching of SF 6 /He discharges (high He percentage)
730 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010 and as a function of separated and combined beams of XeF 2 and Ar (Refs 15,16). Etch rate is very low at high He concentration of 40% for many reasons, such as the reduction of fluoride atoms, the oxide and polymerized layers production. Polymerization usually resulted depending on some other plasma parameters, deposition occurred rather than etching 15, as shown in Fig. 15. 4 Conclusions The visualization of I-V and I-P characteristics shows that the electrical discharges plasmas are operated in abnormal region which is effective parameter of surface processing of Si wafer. Results of Ar dc-glow discharges can be used for surface cleaning as primary step to etch processing of other gases. SF 6 plasma were observed to etch Si with high rate while mixing with low percentage of He gas leads to uniform and stable plasma without increasing the etch rate rather than SF 6 pure. References 1 Lieberman M A & Lichtenberg A J, Principles of Plasma Discharges and Materials Processing (John Wiley, New York), 1994, pp 8-10. 2 Chen F, Introduction to Plasma Physics and Controlled Fusion, Plenum Press (New York), 1984, p 13. 3 Brown S C, Introduction to Electrical Discharges in Gases (John Wiley, New York), 1966, p 301. 4 Manos Dennis, M. & Flamm Daniel, L. (eds), Plasma Etching An Introduction, (Academic Press, CA), 1989, pp 25-45. 5 Joseph C, in Handbook of Plasma Processing Technology (eds Rossnagel Stephen, Cuomo Jerome, J. and Westwood William, W.) (Noyes Publications, Park Ridge, N J), 1990, pp 140, 196. 6 Ledernez L, Olcaytug F, Yasuda H & Urban G, ICPIG 29 th, 2009, p 12. 7 Bersin R L, Solid State Technol, May (1976) 32. 8 Chapman B, Glow Discharge Processes: Sputtering and Etching (John Wiley & Sons, New York), 1980, p 299 9 Manos D M & Flamm D L, Plasma Etching: An Introduction (Academic Press, New York Manos), 1989, p 29 10 Bogaerts, Neyts E & Gijbels R, Spectrochimica Acta Part B, 57 (2002) p.609. 11 Flamm D L, Donnelly V M & Mucha J A, J Appl Phys, 52 (1981) 3633. 12 Vasile M J & Stevie F A, J Appl Phys, 53 (1982) 3799. 13 Yan J D, Fang M T C & Liu Q S, IEEE Dielectrics and Electrical Insulation, 4 (1997) 1. 14 Bondur J A, J Vac SciTechnol, 13 (1976) 1023. 15 Mogaband C J & Evinstein H J L, J Vac Sci Technol, 17 (1980) 3. 16 Coburn J W & Winters H F, J Apply Phys, 50 (1979) 3189.